Why?
To burn shit?
There really is no reason you need a 4W portable (near) invisible wavelength laser.
I would strongly recommend you stay away unless you have a legitimate scientific use for it and know and understand the dangers and how to safely operate it.
It is absolutely doable, but not many have done it.
You won't find anyone who has built one selling to someone they don't trust. It is a very dangerous item.
I see.
Something that would suit your application better and be easier to control would be a fiber coupled laser.
The laser is directed through a fiber optic which can be flexible and easier to hold than a large laser.
Also, 4W is a lot of power. In my opinion, too much for engraving wood. In the right circumstances you could end up getting a flame from the wood surface.
2W would be the most I would go with if doing it by hand.
Also, anything in this power range, especially fiber coupled, is going to be quite expensive.
If you haven't already, I would look into alternative methods for cost-effectiveness.